|Moderated by: Greg Fletcher|
|I recently rebuilt 13046 with a stroker crank conversion and 104 & 107 cams with 45 Delorottos and 10.5:1 pistons and ported polished rebuilt head . I measured the compression after the rebuild and have approx 1000 miles on the rebuilt engine and got following results.
#1 - 180 psi
#2 - 180 psi
#3 - 180 psi
#4 - 175 psi.
These readings were taken a cooled down engine with the all plugs removed, fuel pump shut off and the throttle open. Do I also need to compare readings to a warmed up engine?
I saw the specs from JH and Lotus for the stock 907 engine but figured since this was a stroker crank upgrade, these stock numbers were not applicable. What should my readings be?
Your comments are appreciated.
|Those readings look great! I got 155 psi with flat top pistons. Your valves and rings are sealing well!
|Cylinder pressures vary with compression ratio, not stroke length/ displacement.
It's normal practice to check cylinder pressures on a hot engine, with the plugs removed, the throttle held wide open, and the engine cranking over at 200 rpm or better. Lotus' pressure specs are based on such a practice; however, Jensen-Healey specified checking pressures on a cold engine. So, on a low compression 907, we have the luxury of two compression pressure specs, one cold and one hot. Hot is the most indicative of the engine's operating condition, but cold may be all you can get if the engine isn't running.
Compression Pressure Specs:
907LC .... 8.4:1 .... 110-130 psi COLD = Jensen-Healey spec
907LC .... 8.4:1 .... 150-170 psi HOT.. = Lotus Spec
907HC .... 9.5:1 .... 165-185 psi HOT
912LC .... 9.44:1 ... 165-185 psi HOT
912HC .. 10.9:1 ..... 170-190 psi HOT
Regardless of the pressure, all cylinders should be within 10 psi of one another.
The modern (since Jan 1993) composite head gasket's compressed thickness is half a millimeter (0.020") thicker than that of the old steel-asbestos-steel gasket. With all else being the same, the result is about a half point drop in compression ratio... ie, 8.4:1 becomes 7.9:1.
Last edited on 12-14-2017 12:29 am by Esprit2